I don't see why a large family would be an excuse. You need a large vehicle because you chose to need a large vehicle?
Having more people pay tax does not mean there is a net increase in tax collected.
Some of the children from a large rural families will move to cities and make careers that pay lots of taxes. Cities almost always have lower birthrates than countryside.
Even just maintaining replacement rate fertility requires an average of 2.1 children TFR, which means some families would need to have 3 or 4 children to get there since other families will have only 2 or 1 child.